Smartcards are secure devices routinely used in a wide variety of applications. Smartcards often include integrated circuits (such as electronic circuitry and memory) embedded in small portable carriers. The integrated circuits in the smartcards are typically capable of receiving, transmitting, and storing information.
Smartcards may be broadly divided into different categories, including smartcards with contacts and contactless smartcards. Smartcards with contacts generally communicate after being physically connected via contacts to a smartcard reader or writer. Contactless smartcards generally communicate using wireless mechanisms, such as radio frequency (RF) communications. Contactless smartcards may have greater durability than smartcards with contacts because contactless smartcards are not physically connected to an external device. Also, contactless smartcards often receive operating power by rectifying an alternating current (AC) produced through electromagnetic induction to generate a direct current (DC) voltage. The DC voltage is used to activate and operate the integrated circuits in the contactless smartcards. This helps to eliminate the need for batteries or other internal power supplies in the contactless smartcards, which also helps to reduce the size and cost of the contactless smartcards.
Contactless smartcards have many broad applications. Information stored within and transmitted by a contactless smartcard can be used to perform various functions. These functions could include controlling security access to a restricted area, performing personal identification during commercial transactions, identifying products to ascertain origin and quality, and identifying animals to conduct experiments. One commonly used type of contactless smartcard is a commutation card. A commutation card typically represents a type of smartcard used to gain access to a form of transportation (such as a train or subway). The commutation card typically allows a user of the card to pass through a ticket gate without requiring the user to physically insert the card into a card reader.
FIG. 1 illustrates a typical contactless smartcard 100. In this example, the typical contactless smartcard 100 includes an antenna terminal 102, a resonance capacitor 104, a rectifier circuit 106 producing a power supply output 108, an analog circuit 110, a digital circuit 112, a memory control circuit 114, a memory circuit 116, and a smoothing capacitor 118.
The antenna terminal 102 is typically formed of antenna coils and communicates with a smartcard reader or writer via an electromagnetic field. The antenna coils are often thin wire coils that wrap around an inner perimeter of the smartcard 100 in one or more turns, forming the antenna terminal 102. A smartcard reader or writer typically radiates a high-frequency magnetic field (such as a 13.56 MHz field) via its own antenna. When the antenna terminal 102 of the contactless smartcard 100 is located within the magnetic field, electromagnetic waves from the smartcard reader or writer are received by the antenna coils, producing an AC signal through electromagnetic induction.
The resonance capacitor 104 is coupled to the antenna terminal 102 in parallel so as to resonate at the frequency of the electromagnetic waves. The rectifier circuit 106 converts the AC signal from the antenna terminal 102 into a DC voltage (the power supply output 108). The smoothing capacitor 118 is connected in parallel to the rectifier circuit 106 so as to smooth the power supply output 108. The power supply output 108 is provided to the analog circuit 110, the digital circuit 112, the memory control circuit 114, and the memory circuit 116.
The analog circuit 110 typically includes components such as a demodulator circuit and a modulator circuit. The demodulator circuit is often used to decode received data transmitted over a carrier of the electromagnetic waves. The modulator circuit is often used to superimpose a transmission signal generated by the digital circuit 112 on the carrier of the electromagnetic waves.
The digital circuit 112 typically includes components such as a central processing unit (CPU) for performing various digital signal processes. The memory control circuit 114 typically controls the operation of the memory circuit 116. The memory circuit 116 is typically used to store and facilitate retrieval of data. The memory circuit 116 could, for example, represent a nonvolatile memory.
The power provided by the rectifier circuit 106 is often proportional to the strength of the magnetic field imposed on the antenna terminal 102. At the same time, the strength of the magnetic field on the antenna terminal 102 is inversely proportional to the distance of the antenna terminal 102 from the smartcard reader or writer. As a result, the strength of the magnetic field often varies very greatly in working conditions. At very short distances between the smartcard reader or writer and the antenna terminal 102, a voltage induced in the antenna terminal 102 could exceed the required supply voltage. This is generally referred to as an “overvoltage condition.”
FIG. 2 illustrates a typical power supply circuit 200 used in the typical contactless smartcard 100 to derive power from a smartcard reader or writer and to provide power to other components of the smartcard 100. In this example, the power supply circuit 200 includes the antenna terminal 102, the resonance capacitor 104, the rectifier circuit 106, two smooth capacitors 118a-118b, two zener diodes 202a-202b, and a voltage regulator 204. The rectifier circuit 106 in FIG. 2 is shown as a full-wave rectification circuit that employs four diodes 206.
In the power supply circuit 200 of FIG. 2, without any protection against overvoltage conditions, the voltages denoted VAC0 and VAC1 could easily exceed the typical breakdown voltages of semiconductor devices within the smartcard 100. Therefore, typical power supply circuits 200 often attempt to limit or clamp the voltages VAC0 and VAC1 below the breakdown voltages. In order to clamp the voltages VAC0 and VAC1, zener diodes 202a-202b are coupled to the voltages VAC0 and VAC1, respectively. When either voltage VAC0 or voltage VAC1 is above a certain threshold, the associated zener diode triggers and clamps the voltage VAC0 or VAC1 below the breakdown voltage. However, zener diodes are often not readily available in standard process technologies, such as the Complimentary Metal Oxide Semiconductor (CMOS) process technology. Also, zener diodes often lack flexibility in being adopted into other systems.